REPORT ON CORALS—HYDROCORALLIN-®. 29 
zooids, the absence of all trace of mesenteries, the apparent septa present in the tentacles, 
the presence of the nematocysts of the form peculiar to Hydrozoa, and in fact every item 
of histological structure, point irresistibly to the same conclusion. Professor Agassiz 
considered the Millepores to be allied to the Hydractiniz, and Claus remarks on their 
resemblance in some points to the Corynide. Both Hydractinia and Podocoryne 
resemble Millepora in having a ccenosare which forms a continuous encrusting layer ; 
and in essential structure the ccenosare of these two genera seems closely to resemble 
that of Millepora. Mr Carter’ has described a species of Hydractinia from the Guinea 
coast, H. calearea, which has a hard caleareous ccenosteum. The genus Podocoryne 
(Sars) has a “hydrophyton consisting of a continuous adherent expansion formed by 
adnate inosculating canals, the deeper part, with its component canals, invested by a 
chitinous perisarc, while a layer of naked ccenosarc spreads over the free surface.” In 
Millepora the canals are not adnate, being separated by the stout trabecul of calcareous 
matter which here take the place of the chitinous perisarc. The layer of naked ccenosare 
on the surface is probably homologous with the layer in the ccenosare of Millepore 
described in the present paper as the superficial layer of the ectoderm. The structure 
of the ccenosare of Hydractinza is essentially similar to that of Podocoryne. Distinctive 
features in the ccenosare of Millepora are the presence in it of the pore-like excavations 
into which the zooids are retracted, the presence of large main branching canals, and the 
formation of successive superposed layers of ccenosarc, and consequent formation of lines 
of growth and tabule in the calcareous skeleton. In having zooids of two kinds, 
mouth-bearing and mouthless, the Millepores resemble Hydractinia echinata, which 
bears likewise alimentary (gastrozooids) and spiral mouthless zooids (dactylozooids). In 
the form of the zooids, however, and shape and arrangement of the tentacles, and in 
the nature of the nematocysts,” Millepora seems to resemble such a form as Gemmaria 
implexa. The real athnities of Millepora amongst the Hydroids cannot, however, be 
determined until the mode of reproduction is discovered. 
It is a remarkable fact that the ccenosteum of Millepora seems undoubtedly to be 
generated by the ectoderm. It is thus not homogenous with the corallum of Anthozoa, 
which is developed from the mesoderm, as appears certain in the latest accounts of 
the matter from M. Lacaze-Duthiers’® researches on Astroides calycularis, and from 
those of Kowalewsky* on Astrea and on Aleyonium digitatum. I have, for this reason, 
1H. J. Carter, F.R.S., On the Close relationship of Hydractinia Parkeria and Stromatopora, with Descriptions of 
new species of the former, both recent and fossil, Ann: and Mag. Nat. Hist., vol. xix. p. 44, 4 ser., 1877. 
2 Tt would seem that a classification and nomenclature of the various forms of thread-cells is much needed, since 
these forms appear to be of classificatory value in the Coelenterata. Certain forms are peculiar to Hydroids, ¢.g., others 
to Aleyonaria. : 
3H. de Lacaze-Duthiers, Développement des polypes et de leur polypier, Comptes Rendus, 1873, t. Lxxvii. 
(Hoffman und Schwalbe, Jahresbericht, 1875). 
+ A. Kowalewsky, Untersuchungen tiber die Entwicklung der Ccelenteraten, Nachrichten der kaiserlichen 
Gesellschaft der Freunde der Naturerkenntniss, der Anthropologie und Ethnographie, Moskau, 1873. (Ibid.) 
